The Collaborative International Dictionary of English v.0.48:

Ruta-baga \Ru`ta-ba"ga\, n. (Bot.)
   A kind of turnip commonly with a large and long or ovoid
   yellowish root; a Swedish turnip. See Turnip.
   [1913 Webster]

WordNet (r) 3.0 (2006):

rutabaga
    n 1: the large yellow root of a rutabaga plant used as food
         [syn: rutabaga, swede, swedish turnip, yellow
         turnip]
    2: a cruciferous plant with a thick bulbous edible yellow root
       [syn: rutabaga, turnip cabbage, swede, Swedish
       turnip, rutabaga plant, Brassica napus napobrassica]